This invention relates to a process for the purification of glyoxal via the preparation of commercial crude glyoxal solutions to pure, aqueous glyoxal solutions.
Glyoxal is commercially prepared by oxidation of acetaldehyde with aqueous nitric acid. The raw product obtained thereby contains unreacted starting materials, acetaldehyde and nitric acid and a number of by-products all of which renders complete separation considerably difficult.
By-products included acids volatile at elevated temperatures, for example acetic acid and formic acid, non-volatile acids, for example glyoxal acid, glycolic acid and oxalic acid and organic electrolyte and other impurities. These cause a more or less heavy discoloration of the product.
The main part of the volatile acids, however, can be removed by distillation and concentration of the solution, but there is obtained a crude glyoxal which still contains residual acid which cannot be removed by this method. According to prior methods, these acids are neutralized with carbonates of a metal of Group II, especially calcium carbonate. The separation of calcium salts in solid form is, however, incomplete, so that the glyoxal solution also contains calcium ions. Glyoxal solutions purified in this way have a dark yellow color.
It has been proposed to remove the non-volatile acids with anion exchangers, for example according to the method of German Patent 1,154,081. A comparison of the acid content of glyoxal solutions purified by distillation with the capacities of an anion exchanger (.about.1,5 val/1) shows that the same volume of the exchanger resin is required for the deacidification of a certain volume of 40% crude glyoxal solution. With an anion exchanger resin, both after saturation to remove the glyoxal solution and after regeneration with soda lye, a 6- to 10-fold volume of purification liquid is needed which makes the efficiency of this process very poor.
A drawback to the electrolytical dialysis process is also the high water use which is described in German Patent 1,618,283. At a residence time of 20 hours, a glyoxal output of 80 to 90% is reached.
According to the process of Russian Patent 168,670, at the tetraacetal is produced from the crude glyoxal and ethanol and is then hydrolyzed after purification by distillation with acid cation exchangers. According to experiments and information in the literature (J.A.C.S. 77, 1285 (1955)), however, the formation of acetal even with excess alcohol takes place in a very unsatisfactory manner.
According to another process, described in German Offenlegungsschrift 2,159,975, the production and use of glyoxal hemiacetals as an intermediate product for producing of acid-free, aqueous glyoxal solutions is described. Glyoxal demiacetals are isolated from technical glyoxal solutions and purified by reaction with water immiscible alcohols. In a three-stage hydrolysis with boiling water only 80% as hydrate can be achieved. The solution still contains large traces of the immiscible alcohol. A drawback to this process is the incomplete separation of the glyoxal from its solution which cannot be corrected by using an excess of the immiscible alcohol.